Membrane-based sensor device with non-dielectric etch-stop layer around substrate recess

ABSTRACT

A sensing device has a semiconductor substrate with an opening and a membrane spanning the opening. A heater is arranged on the membrane. To reduce the thermal conductivity of the membrane, a recess is etched into the membrane from below.

TECHNICAL FIELD

The invention relates to a sensor device having a semiconductorsubstrate with an opening, a membrane extending over said opening and aheater located on the membrane. The invention also relates to a methodfor manufacturing such a sensor device.

BACKGROUND ART

Various sensor devices require a heater. One group of such devices isformed by flow sensors, where changes in the thermal distribution aroundthe heater are used to measure a fluid flow. Such a device is e.g.disclosed in US 2007/0241093. Another group of such devices comprisesgas sensors, where the heater is used to heat a sensing material, suchas a metal oxide, to an operating temperature. A device using such a hotplate is e.g. disclosed in WO 95/19563.

This type of devices can be integrated onto a semiconductor substrate,in which case, the heater is advantageously arranged on a membrane overan opening in the semiconductor substrate, thereby reducing the thermalloss as compared to devices where the heater is arranged over the bulkof the substrate material. Arranging the heater on a membrane has aseries of advantages, such as reducing power consumption, increasingsensitivity and reducing the time required for switching on the device.

DISCLOSURE OF THE INVENTION

According to a first aspect of the present invention, a sensor devicecomprises a semiconductor substrate (such as a silicon substrate) havinga top and a bottom surface and an opening extending between the top andbottom surfaces. A batch of material layers, e.g. comprising structureddielectric and metallic or semiconducting layers, is arranged on the topsurface of the substrate, e.g. in order to form conducting leads andother electrical and electronic components of the device. Some of thematerial layers extend over the opening of the semiconductor substrate,thereby forming a membrane. Further, a heater is arranged on themembrane.

In addition, the device comprises a recess extending from below into thebatch of material layers at the location of the membrane, therebyreducing the thickness of the membrane and therefore thermal lossesthrough the membrane.

A device of this type can be manufactured according to a second aspectof the present invention by providing a semiconductor substrate having atop and a bottom surface. The mentioned batch of material layers isapplied to the top surface, and a heater is formed in the batch ofmaterial layers by suitable structuring techniques. Further, an openingis etched through the substrate, thereby forming a membrane formed bythe material layers at the location of the heater. In addition, a recessis etched into the bottom side of the batch of material layers, namelyby applying an etching agent through the opening in the substrate. Inthis manner, the thickness of the membrane is reduced.

This design is particularly advantageous when being combined with modernCMOS processes, where typically a very large batch of material layers isapplied to the semiconductor substrate. This batch can have a thicknessof 10 μm or more, even at locations where the metal layers are removed.By forming said recess in the membrane, the thickness of the membranecan be optimized.

Etching the recess from below has the further advantage that it does notaffect the topography of the top surface of the batch of materiallayers, while forming the recess from above would lead to a non-flatsurface of the membrane, which would render the formation of electrodestructures and of other type of surface structures more difficult, inparticular when using photolithography.

Typically, the batch of material layers comprises a plurality ofstructured dielectric layers and a plurality of structured metal layers,such as e.g. known from conventional CMOS devices. It can e.g. bemanufactured by subsequent application of unstructured layers ofsuitable materials and structuring the same using photolithographictechniques.

Further, the batch of material layers advantageously comprises at leastone non-dielectric etch-stop layer, which is structured to extend overthe location of the opening to be formed in the substrate. Thisetch-stop layer is used as an etch-stop when forming the recess into thebatch of material layers. Even though the etch-stop layer is notstrictly necessary (timed etching could be used instead in order to etchthe recess to a certain depth), it is advantageous since it allows tostop the etching-process at a well-defined location.

After etching the recess, at least part of the etch-stop layer maypreferably be removed in order to further reduce the thermalconductivity of the membrane.

The sensor is advantageously a gas sensor or a flow sensor.

Other advantageous embodiments are listed in the dependent claims aswell as in the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described by reference to theannexed drawings, wherein:

FIG. 1 shows a view of a sensor device,

FIG. 2 shows a sectional view of the device,

FIG. 3 shows a sectional view of the device after forming the batch ofmaterial layers,

FIG. 4 shows a sectional view of the device after forming the recess,

FIG. 5 shows a second embodiment of the device with a silicon platearranged at the bottom of the membrane, and

FIG. 6 shows a view of a flow sensor.

Note: The drawings are not to scale.

MODES FOR CARRYING OUT THE INVENTION Definitions

Terms indicating a vertical direction or arrangement, such as “top”,“bottom”, “above” or “below” relate to a frame of reference where thebatch of material layers forming the membrane are arranged on top, i.e.above, the substrate. In other words, the substrate is arranged, bydefinition, below the material layers and the membrane is located on topof the opening extending through the substrate.

The term “lateral” is used to describe directions parallel to the topand bottom surfaces of the semiconductor substrate.

A “dielectric” material is a non-conductive, non-metallic material, inparticular an oxide or nitride, such as SiO₂ or SiN.

A “non-dielectric” material is a metal or a metalloid.

An “SOI-structure” (“Silicon on Insulator”) is a structure comprising ahandle substrate of silicon, a silicon layer, and an insulator layerarranged between the handle substrate and the silicon layer.

The device:

FIG. 1 shows a gas sensor adapted to generate a signal indicative of theconcentration of at least one gaseous analyte in a gaseous carrier, suchas alcohol in air. It comprises a semiconductor substrate 1. A sensormaterial, whose electrical properties depend on the concentration of theanalyte, is applied to substrate 1 in a patch 2. For example, patch 2consists of a granular layer of SnO, whose electrical resistance dependson the presence and concentration of various compounds in thesurrounding atmosphere. This type of device is e.g. described by WO95/19563.

Patch 2 is in electrical contact with at least a pair of interdigitalmetal electrodes 3, which are connected to processing circuitry 4.Processing circuitry 4 is integrated on semiconductor substrate 1 andcan e.g. comprise active components, such as transistors, at least oneamplifier, at least one analog/digital converter, and/or interfacecircuitry, etc.

The sensor device further comprises a heater 5 positioned at thelocation of patch 2 in order to heat patch 2 to its operatingtemperature, which, for SnO, is e.g. typically at least 300° C.

FIG. 2 shows a sectional view of this type of device. As can be seen,semiconductor substrate 1 comprises a bottom surface 7 (cf. FIG. 1) anda top surface 8. A batch 9 of material layers is applied to top surface8 and typically comprises a plurality of structured dielectric layersand a plurality of structured metal layers as used in standard CMOSprocesses.

Typically, the metal layers, which are generally designated by referencenumbers 10 a-10 f, are of aluminum or copper, and they are structured toform leads and other electrical components. In FIG. 2, they are onlyshown schematically. The metal layers 10 a-10 f are separated bydielectric layers, typically SiO₂ layers, which are generally denoted byreference number 11.

Part of the layers of batch 9 extend over an opening 12 in semiconductorsubstrate 1 and form a membrane 13. Membrane 13 can have circular orrectangular cross section or any other suitable cross-section.

Advantageously, and in order to reduce the thermal conductance ofmembrane 13, none of the aluminum or copper metal layers 10 a-10 fextends into membrane 13.

Batch 9 further comprises a layer 14 of SiN under tensile stress, whichextends at least over membrane 13 and is anchored laterally outsidemembrane 13. The tensile stress in layer 14 is at least sufficientlylarge to exceed the compressive stress in the rest membrane 13, whichleads to a total tensile stress in the membrane. As described in U.S.Pat. No. 7,154,372, such a tensile layer can be used to prevent themembrane from buckling.

Heater 5 is formed by structuring a metal layer into at least one metalconductor, which is located on membrane 13. Advantageously, heater 5 isformed by a tungsten conductor. As seen in FIG. 1, the metal conductorcan e.g. follow a meandering path. The layer of heater 5 is arranged onSiN layer 14.

A SiO₂ layer 15 is arranged on top of the layer of heater 5 andelectrically insulates the same from a further metal layer forming theelectrodes 3.

A protective dielectric layer can be applied to the top of the device(not shown).

As can be seen in FIG. 2, at the location of the membrane the devicefurther comprises a recess 17 extending from below into the batch 9 ofmaterial layers. This recess has the purpose, as discussed above, toreduce the thickness of membrane 13.

Typically, batch 9 has a thickness of at least 5 μm, in particularbetween 6 and 15 μm. Recess 17 has a vertical depth of at least 1 μm, inparticular of at least 3 μm.

The cross section, i.e. the lateral extension, of recess 17 isadvantageously equal to the cross section of the upper end of opening12, such that it spans the whole membrane, and the membrane thereforehas a reduced thickness everywhere. Alternatively, the cross sectionarea of recess 17 may be at least 80% of the cross section area of theupper end of opening 12.

Membrane 13 preferably has sufficiently large lateral extensions toprovide a thermally insulated location for receiving patch 2.Advantageously, membrane 13 may cover an area of at least 0.1 mm², inparticular at least 0.2 mm².

For reasons that will become apparent from the description of themanufacturing process below, the sensor device further comprises atleast one etch-stop layer 20, or, rather, residual remains thereof,arranged in a ring around recess 17. Etch-stop layer 20 is of anon-dielectric material, in particular of aluminum or copper. The ringof etch-stop layer 20 has a lateral width W between 1-20 μm in order tobe able to compensate for positioning errors in the etching processdescribed below. The shape of the ring formed by edge-stop layer 20depends on the shape of recess 17.

As can be seen, etch-stop layer 20 is advantageously positioned at aheight between two of the metal layers 10 a-10 f, i.e. at anintermediate height of batch 9, namely at such a height that thedistance between etch-stop layer 20 and the top of batch 9 correspondsto the desired thickness of the membrane.

Manufacturing Process:

FIGS. 3 and 4 show a method for manufacturing the sensor device.

In a first step, silicon substrate 1 is covered, at its top surface,with the dielectric layers 11 as well as with the metal layers 10 a-10 fin a series of steps as known in the art of semiconductor devicemanufacture. For example, a first SiO₂-layer is applied to top surface8, then the first metal layer 10 a is applied onto the first SiO₂-layerand structured using photolithography. Then, a second SiO₂-layer isapplied, and the second metal layer 10 b is applied thereto andstructured, etc.

Between two of these steps, etch-stop layer 20 is applied and structuredto extend over the area of future opening 12 and slightly beyond. Forthe reasons mentioned above, at least one first metal layer (in theembodiment of FIG. 3 the metal layers 10 a-10 c) is added to batch 9 oflayers before etch-stop 20 layer is formed, and at least one secondmetal layer (10 d-10 f) is added to batch 9 of layers after etch-stoplayer 20 is formed.

After applying the metal layers 10 a-10 f, etch-stop layer 20 and thedielectric layers 11, the layer for heater 5 is applied and structured,followed by the application of tensile layer 15 and the electrodes 3.This results in a device as shown in FIG. 3.

In a next step, opening 12 is etched out from the bottom side 7 ofsemiconductor substrate 1, e.g. using a plasma-process (such as deepreactive ion etching) or a wet process (such as anisotropic etchingusing KOH) after application of a photolithographic mask to bottom side7. This step uses the bottom surface of the dielectric layers 11 as anetch-stop.

In a next step, recess 17 is etched. In this step, etch-stop layer 20acts as an etch-stop. If recess 17 is to have the same lateral extensionas opening 12, no masking is required in this step.

The resulting device is shown in FIG. 4.

In a next step, the accessible part of etch-stop layer 20 can optionallybe removed by etching. A residual ring of the etch-stop layer 20 ofwidth W surround recess 17, as described above, remains in the device.

Now, patch 2 of the sensing material can be applied to the top ofmembrane 13.

ALTERNATIVE EMBODIMENT

FIG. 5 shows an alternative design of the device with a silicon plate 30arranged at the bottom of membrane 13. Silicon plate 30 is not connectedto substrate 1, i.e. there is a gap between silicon plate 30 andsubstrate 1, which extends all around silicon plate 30 and is formed byopening 12 and recess 17. Recess 17 has annular shape.

The purpose of silicon plate 30 is to provide a uniform temperaturedistribution at the location of patch 2.

Plate 30 can have a thickness equal to substrate 1, in which case thedevice can be manufactured as described above with the sole differencethat opening 12 has annular shape, thereby forming plate 30 fromsubstrate 1.

Alternatively, and as shown in FIG. 5, plate 30 can have a thicknesssmaller than the thickness of substrate 1. To manufacture a sensor ofthis type, substrate 1 can be formed by an SOI structure. In otherwords, an SOI-wafer is used for manufacturing substrate 1. Such anSOI-waver comprises a handle substrate 1 a, an insulating layer 1 b (inparticular SiO₂) arranged on substrate 1 a, and a monocrystallinesilicon layer 1 c on top of insulating layer 1 b. Typically, thethickness of silicon layer 1 c is much smaller than the thickness ofhandle substrate 1 a. This type of SOI-waver is known to the skilledperson.

The batch of layers 9 is integrated on top of silicon layer 1 c.

For manufacturing opening 12 and recess 17, the device is again etchedfrom its bottom side. In a first step, a first part 12 a of opening 12is formed, using insulating layer 1 b as an etch stop. A mask is thenapplied on insulating layer 1 b at the location of the future plate 30,and insulating layer 1 b is removed outside this mask. Then, an annularsecond part 12 b of opening 12 is formed by etching off silicon layer 1c where it is not masked, thereby forming plate 30 from silicon layer 1c. Second part 12 b forms the annular gap extending all around plate 30between plate 30 and substrate 1. Finally, recess 17 is formed byetching from the side second part 12 b of opening 12 as described above.

Flow Sensor:

As mentioned, the sensor device can also be a thermal flow sensor, e.g.of the type described in U.S. 2007/0241093, which has a heater arrangedon the membrane. A fluid flowing over the membrane distorts the thermalfield generated by the heater, which in turn can be detected by one ormore suitable temperature sensors arranged on the membrane.

FIG. 6 shows such a flow sensor having an elongate heater 5 arranged onmembrane 13, as well as at least one, in particular two temperaturesensors 31 a, 31 b on membrane 13. The two temperature sensors 31 a, 31b are arranged on both sides (upstream and downstream) of heater 5. Thetemperature sensors 31 a, 31 b in the shown embodiment are thermopiles,as described in U.S. 2007/0241093. The device can be used to measure aflow F in a direction perpendicular to the elongate axis of heater 5.

Notes:

In the embodiment described in reference to FIGS. 1-5, the sensor devicewas a gas sensor having a metal oxide, in particular SnO, as sensingmaterial. The device can, however, also be a gas sensor using anothersensing material as known to the skilled person.

The sensor device may also be any other type of device where a heater isarranged on a thin membrane.

While there are shown and described presently preferred embodiments ofthe invention, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpractised within the scope of the following claims.

The invention claimed is:
 1. A sensor device comprising a substratehaving a top and a bottom surface and an opening extending between saidtop and bottom surfaces, a batch of material layers applied to said topsurface, wherein at least some of said material layers extend over saidopening for forming a membrane, a heater arranged on said membrane,wherein said sensor device comprises a recess extending from below intosaid batch of material layers at a location of said membrane, whereinsaid batch of material layers further comprises at least onenon-dielectric etch-stop layer extending in a ring around said recess,and wherein said ring is at level with a to surface of the recess. 2.The sensor device of claim 1 wherein said recess has a depth of at least1 μm.
 3. The sensor device of claim 2, wherein said recess has a depthof at least 3 μm.
 4. The sensor device of claim 1 wherein said recesshas a cross section area of at least 80% of a cross section area of anupper end of said opening.
 5. The sensor device of claim 4, wherein saidrecess has a cross section equal to the cross section of the upper endof said opening.
 6. The sensor device of claim 1 wherein said batch ofmaterial layers comprises a plurality of structured dielectric layersand a plurality of structured metal layers.
 7. The sensor device ofclaim 1, wherein said ring has a width between 1 and 20 μm.
 8. Thesensor device of claim 7, wherein said non-dielectric etch-stop layer isof aluminum or copper.
 9. The sensor device of claim 1 wherein saidmembrane comprises at least one SiN-layer under tensile stress.
 10. Thesensor device of claim 1 further comprising at least one patch of asensing material arranged on said membrane, and electrodes contactingsaid sensing material.
 11. The sensor device of claim 10, wherein saidat least one patch of a sensing material is a metal oxide.
 12. Thesensor device of claim 10, wherein said electrodes are metal electrodes.13. The sensor device of claim 1 wherein said heater comprises at leastone metal conductor.
 14. The sensor device of claim 13, wherein said atleast one metal conductor is a tungsten conductor.
 15. The sensor deviceof claim 1 further comprising processing circuitry integrated on saidsubstrate.
 16. The sensor device of claim 15 wherein said processingcircuitry comprises at least one amplifier, analog/digital-converter orinterface circuitry.
 17. The sensor device of claim 1 further comprisinga silicon plate arranged at a bottom of said membrane, with a gap formedaround said plate between said plate and said substrate.
 18. The sensordevice of claim 17, wherein said substrate is an SOI structure having asilicon handle layer, an insulating layer and a silicon layer, with saidinsulating layer arranged between said silicon handle layer and saidsilicon layer.
 19. The sensor device of claim 1, wherein said sensordevice is a gas sensor or a humidity sensor.
 20. The sensor device ofclaim 1 wherein said sensor device is a flow sensor comprising at leastone temperature sensor arranged on said membrane.